1. Field of the Invention
The present invention relates to a developing roller used in contact with a photoconductive member which is assembled in an apparatus utilizing electrophotographic techniques, such as a copying machine, a printer, or a facsimile receiving unit. The present invention also relates to a developing device using the developing roller.
2. Description of the Related Art
For charging a photoconductive member or visualizing an electrostatic latent image, an electrophotographic apparatus, e.g., a copying machine, a facsimile or a printer, generally employs an elastic roller having conductivity (electrical resistance) that is in the semiconductor range of 103-1010xcexa9 and is suitable for its objective function. In an electrophotographic apparatus utilizing a one-component developing method, for example, an electrostatic latent image is visualized for development with a developer (toner) carried to a photoconductive member (drum) through developing rollers that are arranged in a pressure contact state with each other. An elastic roller for use in such an electrophotographic apparatus is required to easily deform and have a good restoring ability from a deformed state, because the elastic roller must be brought into pressure contact with the photoconductive member over a predetermined width or it must hold thereon the developer in the form of a thin layer with the aid of a blade or the like. The elastic roller is also required to have uniform conductivity at an appropriate level in the range of 104-108xcexa9 so that a developed image will not undergo unevenness.
To satisfy those requirements, a roller comprising a core shaft and an elastic body layer, which is made of a silicone rubber material given with conductivity and is formed around the core shaft, has hitherto been employed for the above-stated type of roller, particularly a developing roller concerned with the present invention.
However, such a conventional developing roller has the following drawbacks due to characteristics of its elastic body layer.
When employing elastic rubber to form the developing roller, if a rubber having a low hardness is selected to obtain a good ability of following to the surface of the photoconductive member, contamination of the photoconductive member would occur sometimes. Further, the use of a rubber having a low hardness would cause a difficulty in polishing the roller surface, thus resulting in an unsatisfactory roller surface.
In the case of employing a method of forming a surface layer to avoid contamination of the photoconductive member and to obtain a satisfactory surface of the developing roller, if the surface layer contains no conductive powder, the resistance of the overall surface layer would be so increased that the obtained roller may not function as a developing roller. Conversely, if the surface layer contains a large amount of conductive powder, the modulus of elasticity would be excessively increased and a restoring ability from a deformed state (also called a setting ability) required for the developing roller would not be provided. When adding carbon black, particularly, low electrical resistance is obtained, but the required setting ability cannot be obtained because of the surface layer having a high modulus of elasticity. In addition, surface properties and resistance greatly vary depending on a degree of dispersion of the carbon black in the surface layer.
Japanese Patent Laid-Open No. 9-222788 discloses a conductive layer using carbon black having a low structure (oil (DBP) absorption amount of not more than 80 ml/100 g). However, the conductive layer disclosed in Japanese Patent Laid-Open No. 9-222788 has a problem that, because the carbon black used therein has a low effect in applying conductivity, a large amount of carbon black must be mixed to obtain a sufficient conductivity. Consequently, the modulus of elasticity of a rubber composition is excessively increased. Also, it is explained in the Publication that when using carbon black with an oil absorption amount of more than 80 ml/100 g, the amount of carbon black added is relatively small and a difficulty is caused in obtaining satisfactory dispersion and stability of the carbon black in a solution of a coating resin and hence in controlling an appropriate conductivity stably.
The term xe2x80x9cstructurexe2x80x9d used regarding carbon black means an aggregated condition of carbon black particles. The carbon black (CB) is present in a condition where CB particles are fused with each other, which is called an aggregate and compared to a cluster of grapes from a similarity in the concept of configuration. A degree of development into the aggregate is referred to as a xe2x80x9cstructurexe2x80x9d and classified into high, normal (medium) and low levels. A level of the structure greatly affects reinforcement and extrusion characteristics of rubber in which carbon black is mixed, and dispersion, tinting power, viscosity and conductivity resulting when carbon black is used in inks, paints, colored resins, etc.
There is a demand for higher performance required in a developing roller which is employed in an apparatus utilizing electrophotographic techniques, such as a copying machine, a printer, or a facsimile receiving unit, and a particular demand resides in uniform conductivity and a restoring ability from a deformed state (setting ability), which are required for realizing a high resolution. Accordingly, it is an object of the present invention to provide a developing roller, which has a superior setting ability and has a uniform and high conductivity, by improving dispersion and stability of carbon black mixed in a solution of a coating resin, and then forming a resin layer that has a low modulus of elasticity and a high conductivity when coated.
Another object of the present invention is to provide a developing device using the developing roller.
To achieve the above objects, the inventors have carried out intensive researches and studies. As a result, the inventors have accomplished the present invention based on the following findings. By dissolving carbon black, which has a DBP (herein, xe2x80x9cDBP represents dibutyl phthalate.) absorption amount to a nitrogen specific surface area being not more than 0.012 ml/m2, and a ratio of a volatile component to a nitrogen specific surface area being not more than 2.0xc3x9710xe2x88x924 g/m2, in a solvent together with a resin to form a solution of a conductive component, it is possible to obtain an intended resistance value of a coated resin layer and intended dispersion and stability of the carbon black in the solution of the conductive composition. In particular, satisfactory dispersion and stability of the carbon black in the solution of the coating resin can be obtained by selecting a balance in the particle size and the structure level of the carbon black having the DBP absorption amount of more than 80 ml/100 g, which has been regarded as difficult to perform stable control to provide an appropriate conductivity (as explained in Japanese Patent Laid-Open No. 9-222788), so that the carbon black exhibits good dispersion in the solution and has a surface activity (volatile component) keeping the carbon black hard to aggregate again.
Also, in the process of accomplishing the present invention, the inventors have found that, by using the carbon black having such a low reinforcing ability, an increase in the modulus of elasticity is suppressed to be small and a multilayer coating laminated on a roller is avoided from becoming too hard, and that a satisfactory setting ability (restoring ability from a deformed state) can be obtained while sufficiently developing properties of the other layer. Stated otherwise, when a material having a superior setting ability, such as silicone rubber, is used to form an elastic body layer, the specific setting ability of the material can be maintained.
Thus, a developing roller of the present invention comprises a core shaft, an elastic body layer formed around the core shaft and made of rubber as a main ingredient, and a resin layer coated at least on an outer surface of the elastic body layer, wherein the resin layer contains carbon black having a DBP absorption amount of 80 to 110 ml/100 g, a ratio of a DBP absorption amount to a nitrogen specific surface area being not more than 0.012 ml/m2, and a ratio of a volatile component to a nitrogen specific surface area being not more than 2.0xc3x9710xe2x88x924 g/m2.
Also, a developing device of the present invention includes a developing roller for carrying a developer in a state of abutment or pressure contact with an opposing latent-image bearing member for bearing a latent image thereon, the developing roller applying the developer to the latent-image bearing member, thereby visualizing the latent image into a developer image, wherein the developing roller is constituted as above-described.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.